This invention relates generally to endoscopy and, in particular, to a fluid insensitive braking system for use in endoscopes.
The term endoscope shall be used herein in a generic sense to include broadly endoscopes, borescopes, and guide tubes. In many endoscopes the distal end of the insertion tube is capable of being articulated by a steering mechanism that includes a pair of external control wheels coupled to steering cables mounted inside the insertion tube. Rotation of one of the control wheels produces an up or down deflection of the distal tip of the insertion tube while rotation of the second control wheel produces a left or right deflection of the insertion tube tip. As can be seen, by operating the two control wheels, the distal end of the insertion tube can be pointed at a desired target within the range of the instrument or maneuvered through a tortuous path of travel.
The control wheels of the endoscope are typically affixed to superimposed shafts that are mounted on the control handle of the insertion tube. The shafts pass into the housing and are coupled to the steering cables by means of rack and pinion units or the like. It is necessary to apply a holding force to the steering cables in order to maintain the distal tip of the insertion tube at a desired fixed position. Heretofore this was generally achieved by friction pads that applied a friction force against some moving component of the steering section. Fluids, however, can lodge between a conventional brake pad and the moving component of the steering section which changes the frictional characteristics at the interface. A constant torque is needed which is high enough to hold the distal tip of the insertion tube at a desired target position yet low enough to allow the distal tip of the insertion tube to self straighten during withdrawal from a confining space. A constant braking force also provides the user with a definite "feel" for the controls which facilitates rapid and accurate positioning of the insertion tube.
In the case of a medical endoscope, it is extremely important that the insertion tube be immersable in a cleaning fluid for sanitary purposes. This heretofore requires that the friction brake surfaces be completely isolated by fluid tight seals to prevent the pad from becoming wet. It is extremely difficult to properly seal these areas, and oftentimes the seal will deteriorate or break in time with usage. The seals are also expensive to fabricate and maintain and, because of their complexity, can themselves create contamination sites that are hard to keep clean.
U.S. Pat. No. 4,207,873 to Kruy discloses a braking system for an endoscope in which the traditional friction pad brakes are replaced by an incrementing ring having spaced apart indentations formed about its periphery that coact with a pair of pawls mounted in the companion control wheel. Each pawl includes a spring loaded ball which is seated in a set screw that is threaded radially into the hub of the control wheel. The wheel is slidably mounted upon a control shaft adjacent to the incrementing ring which in turn, is affixed securely to the shaft. To actuate the brake, the wheel is moved axially into engagement with the incrementing ring. Each of the spring loaded balls must first snap over a retaining ring before being received in one of the indentations. Once engaged, the spring loaded detent ball places a prescribed holding force against the control wheel.
The incrementing rings and spring loaded balls of the Kruy system are mounted on the outside of the control handle and are thus open to the surrounding ambient. Consequently, the relatively sensitive detent springs acting on the detent balls are exposed to moisture that can find its way into the spring housing set screw and thus cause corrosion of the spring. Any deterioration of the springs will lead to a change in the force exerted upon the steering system. Cleaning of the confined area behind the detent wall is also difficult.